The present invention refers to a sensor and more particularly to one for detecting ambient atmosphere and is related to such devices as gas sensors, humidity sensors, atmospheric pressure sensors and the like.
A conventional gas sensor has been proposed that comprises a metal oxide semiconductor including therein an electrode and a heater serving also as an electrode and capable of sensing ambient atmosphere on the principle that, when the metal oxide semiconductor is heated by the heater, its resistance value drops due to gas absorption by its surface. One problem faced in the application of the gas sensor is that it consumes a large amount of electrical power and is not adaptable to operating with batteries.
To try to solve the above-mentioned problem, a portion projecting into the air such as a bridge structure, cantilever structure or the like, has been provided and a metal oxide semiconductor has been formed thereon to reduce the heating capacity to as low a level as possible and to improve the response time thereby saving on power consumption.
Also a gas sensor having two sensing units with the same construction have been tried, i.e. one which is exposed to ambient atmosphere to detect gas and the another which is entirely shielded from the ambient atmosphere to detect ambient temperature for temperature compensation.
The Japanese publication of unexamined patent applications 3-92754 discloses a complete humidity sensor wherein a first silicon substrate, having two concave portions formed therein, is covered with an insulating protective coat and then provided with two thin film-heating resistors bridged over the respective concave portions thereof and supported thereon.
The above-mentioned sensor is so constructed that heat from the thin-film heating elements of a reduced heat capacity may not directly be transferred to the substrate and the heat's balance may be kept by the spacing formed therein to save on power consumption and to increase its speed of response.
The second substrate has two concave portions formed thereon at places corresponding to those of the first substrate and a notched portion for exposing the pad portion of a thin-film heating resistance.
A bonding glass paste with a low melting point is printed on the substrate by screen-process printing and then a solvent contained in the glass paste is evaporated by temporarily baking it.
The two above-mentioned substrates are opposed to each other in dry air to fill the space of a reference element in such a way that two concave portions of the substrates form two spaces surrounding the respective thin-film heating elements. The substrates are then bonded to each other by heating the glass paste having a low melting point.
The bonded substrates can shut off the reference side's inner space from the outside air and keep a constant ambient atmosphere therein. This ambient atmosphere in the enclosed space may not always be at the proper atmospheric pressure. At the same time, a sensing side inner space is formed in the bonded substrates which has a venting hole formed by utilizing the gap between the joined surfaces of the two substrates.
The newly constructed humidity sensor operates in the following way:
When two thin-film heating elements are supplied with a certain amount of electrical energy, they are self-heated and radiate heat according to the respective amounts of water vapor in their spaces, i.e., the heat conductivity of each space corresponds to the absolute humidity in them and then they reach a constant temperature and represent two different resistance values, the difference of which is detected as the output of an unbalanced potential of a bridge circuit. The absolute humidity can be determined from the detected value.
The above-mentioned -known art, however, has the following drawbacks:
In the forming of the reference space, two semi-conductors are joined by fusing a glass paste with a low melting point, that causes dry air or air with a certain humidity to be heated and thereby to be enclosed as thinned atmosphere in the reference space's chamber. As the result of this, the thin-film heating element cannot sufficiently transfer heat to the charged atmosphere and so its equilibrium temperature rises, making it hard to balance with the sensing portion. Furthermore, considerable time is required to transfer the external ambient temperature to the charged atmosphere thereby affecting the response time. It is also hard to balance the response time with that of the sensing portion.
In addition, the above-mentioned known art provides a shield cover for protecting the sensing and compensating portions from the effect of water used to wash a dicing saw during cutting the semiconductor into the chips. This protection, however, cannot completely protect the sensor because of the electrode pad portion being exposed.